This invention relates to mixed oxide powder mixtures with enhanced reactivity as presursor materials for superconductors and to a process for its manufacture.
In special the present invention relates to precursor powders produced by spray pyrolysis that are then further heat treated such that they show considerable advantage in forming the Pbxe2x80x94Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O 2-2-2-3 superconducting phase over powders produced using other routes.
Superconductors are materials that loose all resistance to the flow of electricity below a critical transition temperature. Today superconducting metals and alloys are being used in a variety of commercial applications in Electronics and Medicine, for example the use of Low Temperature Superconducting Magnets in MRI (Magnetic Resonance Imaging) systems and as Electromagnets in High Energy Physics. The discovery of Superconductivity in the Ceramic Oxide System Laxe2x80x94Srxe2x80x94Cuxe2x80x94O system in 1986 (ref Berdnoz und Mxc3xcller: Z. Physik B 64, 189, 1986) sparked renewed activity in the search for Super-conductivity at temperatures above those only then accessible with liquid helium. Within one year the xe2x80x98High Temperaturexe2x80x99 Yxe2x80x94Baxe2x80x94Cuxe2x80x94Oxide system (HTS) had been discovered in which the critical transition temperature had been raised above 77K (ref Chu: EP341266, 12.01.87), allowing liquid nitrogen to be used as a coolant. Many new High Temperature Superconductors have been discovered and evaluated for potential commercial application over the last ten years, but still the two systems regarded as having the greatest chance of commercial success in the near future are those systems discovered the first, the xe2x80x98Rexe2x80x99(Rare Earth) xe2x80x94Baxe2x80x94Cu Oxides and those based on the Bixe2x80x94Srxe2x80x94Caxe2x80x94Cu Oxide Sytem.
The current market share for the High Temperature Superconductors within the Superconductor Industry is small at ca 3% (15M) of the total (ref BCC Market Research Study: GB-106R, The Superconductor Industry). This is because these Materials and Systems are still in the development and prototype stage, real market growth expected only after the systems have proved themselves to have advantage both in cost and quality (performance and environmental impact). The largest market segment for HTS is currently in elec- tronics and communications where they are used as microwave filters and resonators. Other segments where both the Rexe2x80x94Baxe2x80x94Cuxe2x80x94O and Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O systems are under development include Current Leads to provide low loss energy supply to LTS systems, Magnetic Energy Storage devices able to supply energy when routine supply has been interrupted, Fault Current Limiters to reduce energy supply to a load during a fault, Magnet Systems, Power Cables,Motors and Generators. Wires and Tapes, for use in a number of the afore-mentioned applications, continue to be developed based on the Pbxe2x80x94Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O 2-2-2-3 system. In competition, the IBAD and RaBITs Process using the Rexe2x80x94Baxe2x80x94Cu Oxides are also now attracting more attention (X. D. Wu et al, Applied Physics Letters 65, 1961, 1994).
The current prefered method to manufacture Pbxe2x80x94Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O 2-2-2-3 tapes for energy applications is to use the oxide Powder in Tube method (PIT Method). This process involves the packing of a precursor form into a silver alloy tube that is then subject to repeated mechanical deformation and high temperature thermal treatments. The final product consists of thin filaments of sintered ceramic within the silver-alloy host. During the heat treatment process the precursor form within the silver matrix undergoes conversion to the desired superconducting phase, the Pbxe2x80x94Bxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O 2-2-2-3 phase (ref EP0330305, Jan. 19, 1989; U.S. Pat. No. 4,880,771, Feb. 26, 1988). The quality of the final tapes is dependent upon many process parameters, including the intrinsic nature of the precursor powder, the density of the powder preform, the nature of the silver alloy sheath, the reduction on passing through the drawing die, pressing versus rolling, number of filaments, the calcination atmosphere, the calcination temperature, the calcination time and the heating and cooling cycles. As the processes within the tape during the heat treatments involve chemical conversion to the prefered 2-2-2-3 phase, the intrinsic properties of the precursor powder determine, to a degree, the choice of processing parameters. The conversion of the powder to the 2-2-2-3 phase is thought to proceed via one of two routes, either by growth of the 2-2-2-3 phase from a liquid phase (Flukiger et al, SST, 10, A68, 1997) or by intercalation of calcium and copper oxide layers into the 2-2-1-2 crystal grains already present in the precursor powder (J Jiang, S Abel, SST 10,1997, 678 and references therein).The role of the deformation step in the processing of these tapes is to both texture the grains and to increase the density within the ceramic core. However this step can also introduce cracks and defects in the ceramic which have to be healed during the thermal processing. The liquid phase, produced by melting of particular phases, for example induced through reaction with calcium plumbate, is thought to accelerate phase conversion as well as to both heal these microcracks and remove unwanted grain bounday phases. Therefore the relative amounts of these phases need to be controlled, both in the original precursor itself and through the processing stages to supply enough liquid phase to the system and thereby produce clean grained, well aligned 2-2-2-3 grains at the end of the process. One of the first stages during this conversion is the incorporation of lead from the plumbate phases into the Bi-2-2-1-2 grains. This can be followed by monitoring the change in the phase composition of the material, by for example X-Ray Diffraction. As the reaction proceeds, the calcium plumbate levels decrease as lead enters into the Bi-2-2-1-2 grains. This also results in a change of the lattice type that can be followed with X-Ray Diffraction, changing from a tetragonal unit cell to an orthorhombic one. Too much calcium plumbate is thought to be detrimental to the formation of well textured 2-2-2-3 grains. Excess calcium plumbate can give rise to the growth of undesirable alkaline earth phases during the processing which cannot be removed at a later stage, can disrupt the layering of the 2-2-2-3 grains during mechanical deformation and can decompose, releasing gas, that is trapped momentarily within the silver alloy sheath and results in irrecoverable deformation of the tape. It is also thought that too little calcium plumbate is detrimental in PIT processing because not enough liquid phase can then be supplied during the heat processing stages to heal all the microcracks, clean all the grain boundaries and facilitate the final conversion of residual phases to 2-2-2-3. The final reaction step involves conversion of the lead containing 2-2-1-2 into the 2-2-2-3 phase (R. Flukiger et al, Superconducting Science and Technology 10(1997) A68-A92.
Those powders commonly considered for use as precursors can be manufactured by a number of routes. These include routes starting from solid components, eg the xe2x80x98mix and grindxe2x80x99 process and those routes starting from solutions, eg: citrate gel (U.S. Pat. No. 5,066,636), co-precipitation (U.S. Pat. No. 5,147,848) and spray pyrolysis (GB 8723345).
Although these processes have been applied to the synthesis of superconducting powders there is no systematic procedure for a controlled conversion into the 2-2-2-3 phase of powders produced by any of these routes.
Therfore, there was a need for a method for the synthesis of superconducting powders comprising a controlled conversion into the 2-2-2-3 phase of powders produced.
A solution has been found by a process to manufacture a mixed PbuBivSrwCaxCuy oxide powder in which a mixed-metal solution is sprayed as a fine mist into a heated reactor between 600xc2x0 C. and 1200xc2x0 C. and the collected powder subsequently calcined between 700 xc2x0 C. and 850xc2x0 C. under an atmosphere of between 0.1% oxygen and 21% oxygen for a total heating time at maximum temperature between 4 and 180 hours.
A part of the present invention is a powder prepared according to this process, in which the compositions for this precursor powder is Pb (0.2-0.4) Bi (1.6-2.0) Sr (1.7-2.0) Ca (1.7-2.3) Cu (1.8-3.3) Ox.
The superconducting phase prepared according to the claimed process in which the mixture, as identified by X-ray diffraction techniques, contains one or more than one of the oxides or mixtures of the group (Bi1xe2x88x92sPbs) 1.7-2.4 (Sr1xe2x88x92tCat) 2.6-3.3 Cu 1.8-2.2 Ox phase (abbreviated to 2-2-1-2 phase with s=0-0.4, t=0.2-2.0), (Ca1xe2x88x92mSrm)2PbO4 (where m=0-1), (Sr14xe2x88x92wCaw)Cu24O41 (abbreviated to 14-24 phase with w=0-9), (Ca1xe2x88x92uSru)1xe2x88x92nCuOx (1-1 phase with n=0-0.3, u=0-0.3), (Pb1xe2x88x92xBix) 2.9-3.4 (Sr1xe2x88x92yCay) 4.7-5.0 Cu 0.7-1.2 Oz (abbreviated to 3-3-2-1 phase with x=0-0.4, v=0-0.5), CuO, (Sr,Ca)O, (Bi1xe2x88x92zPbz) 2.0-2.6 (Sr,Ca) 1.4-1.9 Cu 1.8-2.2 Ox (abbreviated to 2-2-0-1 phase with z=0-0.3), (Ca1xe2x88x92pSrp)2CuOx (abbreviated to 2-1 phase with p=0-0.3), Bi6Sr 8.5xe2x88x92rCa 2.5+rOx (abbreviated to 9-11-5 with r=0.0-2.2) and (Bi1xe2x88x92yPby) 1.8-2.3 Sr 1.6-1.9 Ca 1.6-2.0 Cu 2.6-3.1 Ox (abbreviated to 2-2-2-3 phase with y=0-0.4).
A precursor powder prepared according to this process may contain said phases in which the weight % of these phases, as determined using the Rietveld Method described later, are 2-2-1-2 phase: 60-95%, and/or 2-1 phase: 0-24%, and/or 14-24 phase: 0-20%, and/or 9-11-5 phase. 0-18%, and/or (Ca1xe2x88x92mSrm)2PbO4: 0-15%, and/or 3-3-2-1 phase: 0-14%, and/or CuO: 0-11%, and/or 1-1 phase: 0-10% and/or CaO: 0-7%
A precursor powder prepared according to the Process of the present invention in which the lead, has entered into the 2212 grains is also a matter of this application as well as a precursor powder in which the carbon content of said powder is under 500 ppm and the use of the powders prepared for the manufacture of superconducting artefacts.
In summary an Oxide Precursor Powder from the Pbxe2x80x94Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O 2-2-2-3 system can be produced by heat treating powder, produced using the Spray Pyrolysis Process as described in: GB2210605 or EP0681989 between 700xc2x0 C. and 850xc2x0 C. in an atmosphere containing between 0.1% and 21% O2. Heat treatment of the pyrolysis powder under controlled conditions produces a powder with a particular phase composition, that is highly homogeneous and has a small particle size distribution, that is inherently more reactive than powders heat treated in the same way but produced using other processes. This is to be demonstrated in the detailed description below.
It has been found by experiments that the powder produced by the spray pyrolysis process and which was further heat treated to produce a precursor offers significant advantages over those produced by the other routes. In particular the precursor produced from spray pyrolysis powder according to the invention is more reactive, conversion to the 2-2-2-3 phase is faster and thereby saves time and cost, over those produced by the other routes. The precursor produced has a more homogeneous phase distribution and smaller primary particle size than those produced by the other methods.
Under controlled conditions the carbon levels in the powder could also be maintained at a low level, under 500 ppm.
The grain growth on calcining the fine grained pyrolysis powder can often result in powders with primary particles not larger than the raw material components used in the solid state manufacturing process.
An important advantage of the present invention is, that a highly homogeneous powder containing a mix of at least five crystalline phases, each with a small primary particle size is received that reacts by forming the 2-2-2-3 superconducting phase faster than those equivalent powders produced using the citrate gel, co-precipitation and solid state techniques (ref Patents: Liu: U.S. Pat. Nos. 5,066,636, 5,147,848). The main advantage is that the user of such powders requires less time to form the required 2-2-2-3 superconducting phase, saving time and cost in manufacturing their artefacts. This is of particular relevance to the xe2x80x98powder in tubexe2x80x99tape manufactures producing 2-2-2-3 tapes (Sandhage et al, Journal of Materials, Vol 43, No 3, 1991).